1. Field of the Invention
The present invention relates to a communication system using an error correction code, and more particularly to a communication system for transmitting or receiving a block encoded codeword, and decoding into an original codeword using a correlation matrix generated through learning, a correlation matrix learning method, a correlation matrix learning device and its program.
2. Description of the Related Art
In a communication system using an error correction code, a decoding method is conventionally employed in which the transmitting side transmits a block encoded codeword at a predetermined encoding rate, and the receiving side decodes it into an original codeword, using a correlation matrix. This correlation matrix represents correlation between an original pre-coded codeword and a coded codeword, which is determined by learning.
According to a conventional method for learning and obtaining a correlation matrix, first of all a coded codeword and a pre-learned correlation matrix are used to perform a predetermined calculation, thereby obtaining a result containing multiple components. Then, each of the components contained in the calculation result is compared with a preset threshold “±TH”. Based on the comparison, elements of the pre-learned correlation matrix are updated to obtain a post-learned correlation matrix.
Specifically, assuming that the original pre-coded codeword has M bits and the coded codeword has N bits, the correlation matrix has N rows and M columns. When this correlation matrix is multiplied by the coded codeword, the result will contain M number of components corresponding one-to-one to respective bits of the original pre-coded codeword.
A threshold TH is set for each of the components of the original pre-coded codeword in the following way. That is, when a component of the original pre-coded codeword is “+1”, then the threshold is set to “+TH” (TH>0). When a component of the original pre-coded codeword is “0”, then the threshold is set to “−TH”. Each of the components resulting from the calculation is compared with the corresponding threshold “+TH” or “−TH”. When the components resulting from the calculation are to be compared to threshold “+TH”, the components in the corresponding column of the correlation matrix are updated (addition of “±ΔWk” (ΔWk>0)) only when their values are lower than “+TH”. On the other hand, when the components resulting from the calculation are to be compared to threshold “−TH”, the components in the corresponding column of the correlation matrix are updated only when their values are higher than “−TH”. Herein, the sign “±” of the update value for each row depends on the sign “±” of the corresponding bit of the coded codeword.
Thereafter, the learning operation of the correlation matrix is repeated for all of the codewords until update can be no longer performed for the elements of the correlation matrix for all of the codewords. Consequently, if the absolute value of calculation results are greater than or equal to the absolute value of threshold for all of the codewords, the learning operation of the correlation matrix is converged, and correlation matrix is obtained, which can be used for decoding.
When the elements of the correlation matrix have no difference in the calculation results of coding before and after learning operation of the correlation matrix for all of the codewords, the update value is changed from “±ΔWk” to “±ΔWk+1” (ΔWk>ΔWk+1>0). In this way, the update value of the correlation matrix is changed gradually until the learning operation is converged.
Moreover, for all of the codewords, the threshold for learning the correlation matrix is changed gradually such as “TH0, TH1, TH2, TH3, . . . , THn, THn+1, . . . ” (TH0<TH1<TH2<TH3< . . . , THn<THn+1 . . . ) as the learning operation proceeds. Thereby, the correlation matrix is learned while gradually changing a threshold for learning the correlation matrix to search optimal thresholds for all of the multiple codewords, thereby rapidly obtaining an optimal correlation matrix for all of the codewords (e.g., refer to Japanese Patent Laid-Open No. 2002-111515 and No. 2002-314434).
However, the conventional technique had a problem that because a redundant portion for keeping the distance between codewords is collectively added after the original codeword in performing block encoding, the error correction ability is degraded when a burst bit error (burst error) occurs in the redundant portion in transmitting or receiving data on a radio transmission path.